This invention relates to image intensifier tubes of the type used in night vision viewing systems and, more particularly, to an image intensifier tube with reduced veiling glare and a method of making the same.
Image intensifier tubes amplify the amount of incident light they receive and thus provide an increase in light output which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Modern image intensifier tubes utilize a microchannel plate (MCP) which is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron multiplier. When the micro-channel plate is placed in the plane of an electron image in an intensifier tube, one can achieve a gain of up to several thousand. Since each channel in a micro-channel plate operates nearly independently of all the others, a bright point source of light will saturate a few channels but will not spread out over adjacent area. This characteristic of "local saturation" makes these tubes more immune to blooming at bright areas. However, these tubes suffer from a problem known as stray light or "veiling glare".
Stray light is any unwanted unimaged light received by a detector in an optical system. It emanates from bright light rays which are outside the normal field of view. In a lensed optical system, stray light can reflect off of the multiple lens elements causing severe imaging problems by decreasing visibility of low contrast objects. For example, in image intensifier tubes, it results in a loss of contrast by filling in the darker portions of the image.
There have been various attempts to eliminate or reduce stray light including the addition of light absorbing material to the faceplate of the image intensifier tube. For example, a groove was etched between the light input surface of the faceplate and the surface reflecting the stray light, and the groove filled with light absorbing material. Another method included placing a ring of black glass around the outer surface of a clear glass disk. The ring and disk were heated and pressed together to form a unitary structure. These methods have been difficult to perform and have been expensive.
One significant problem involves the formation of the sloping surfaces of the faceplate. In the ring and disk arrangement the sloping surfaces are obtained by the grinding of the black glass ring, thus leaving portions of the sloping surfaces with only a thin layer of the black glass.
In addition, the latter method has caused problems in subsequent processing of the faceplate when photoemissive material is bonded to one surface of the structure. Since the ring and disk are generally formed of two different glasses, bonding is difficult, one reason being the difference in temperatures at which fusing of the cathode material occurs. In order to ease bonding problems, a portion of the black glass adjacent the bonding surface is removed, leaving an unprotected area resulting in 100% internal reflection of stray light off of the surface.
Because the different glasses have different indices of refraction, problems include difficulties with reflection of unwanted stray light.
Another problem arises due to the fact that the material of the black glass ring is transmissive in the 600 nm to 1000 nm (red) spectral region. Since absorption is significantly lower than expected, photons are actually transmitted to the sloping surfaces of the faceplate, and are scattered directly to the cathode. This is a significant problem for image tubes which operate in the 700 nm-900 nm (red) spectral range.
One method of minimizing stray light in the photochromic lens art is by hydrogen reduction of a lens blank. However, in the present art, hydrogen reduction of a faceplate blank results in an extremely thin "skin" layer which is highly transmissive in the red spectral region.
It is therefore an object of the present invention to provide a optical system having reduced light scatter.
It is an additional object of the invention to provide a cathode face plate which reduces the incidence of stray light in an image intensifier tube.
It is a further object of the invention to provide a method for forming such an optical face plate in an easy and economical manner.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides an image intensifier tube having a face plate formed of optical material having an outer surface, one portion of the outer surface being a light receiving surface and another portion of the outer surface being a light transmitting surface, the remainder of the outer surface having a reduced metal oxide material, exhibiting a blackened appearance, included in the optical material for absorbing stray light in the face plate, photoemissive means on the light transmitting surface for emitting electrons in response to light received at the photoemissive means from the light transmitting surface, and a micro-channel plate positioned adjacent the photoemissive means for amplifying the electrons emitted from the photoemissive means.
A method of reducing stray light in a faceplate for an image intensifer tube includes forming a light absorbing layer in the outer surface of the faceplate by causing hydrogen which is pressurized above one atmosphere to react with the oxygen of a metal oxide material of the faceplate to a depth sufficient to reduce stray light which is received in the faceplate and is reflected off of an internal surface thereof.
Additional insight into the present invention may be obtained by reference to commonly owned, copending U.S. patent application Ser. No. 07/233,502, entitled "Reducing Stray Light in Lensed Optical Systems", filed on even date herewith.